Matt's input sounds familiar to me and it is worth to touch each and every cable inside and around the Sleipner BT.
I had a similar experience few month ago where the BT sometimes worked and sometimes not. I measured all cables with the Volt Meter and all the circuits showed me the Voltage as expected in working order. I was close to give up after tested all Voltages, also the BT-Motor including the brushes looked good. Then I started (with the Ohm-Meter) to measure the residences on the cables which should be very low with that sizes of cables. I found a short cable who had a bit higher resistant.
But the connections looked good because the cable and cable-ring terminals where put together with shrink-tubes. I disconnected the cable and pulled on it and the ring-terminal came off. The result you can find on the pictures attached. The cable looked like welded OFF¨! New cables solved the problem.
SV WASABI - Marina di Ragusa (Sicily)
AMEL 54 #55 (2007)